Starting position
The moves 1.d4 d5 2.c4 dxc4 3.e3 b5 — the Queen's Gambit Accepted: Black has taken the pawn on c4 and holds it with the pawn on b5. White undermines the support: 4.a4 c6 5.axb5.
The opponent's mistake
5...cxb5? looks natural: the pawn recaptures and material is level. But the c6 pawn has left the long diagonal, and the path from f3 to the rook on a8 is open.
How the trap works
6.Qf3! — the queen attacks the rook on a8. The rook has nowhere to go: the pawn on a7 and the knight on b8 block it. Black can block with the bishop on b7 or the knight on c6, but neither piece is defended there: 6...Nc6 7.Qxc6+, and after 7...Bd7 8.Qf3 White has a piece for a pawn.
1.d4 d5 2.c4 dxc4 3.e3 b5 4.a4 c6 5.axb5 cxb5 6.Qf3 Nc6 7.Qxc6+ Bd7 8.Qf3
Result
A piece for a pawn — a big advantage for White. Of the Black mistakes covered in the Queen's Gambit lines, this one is the most common.
How to defend (as Black)
Play 5...Bb7: the c6 pawn would stay on the diagonal, and the bishop would defend the rook on a8.